Compression is conventionally carried out by using compression encoding technology to compress image data in order to efficiently transmit and store still image data and moving image data and so forth. The MPEG1 to MPEG4 and H.261 to H.264 systems and so forth are widely used as such compression encoding technology for moving images.
With such encoding systems, encoding processing and decoding processing are carried out after dividing image data constituting the encoding target into a plurality of blocks. Furthermore, in the case of the MPEG4 and H.264 systems, when encoding a target block of an intra frame in order to further raise the encoding efficiency, a predictive signal is generated by using regenerated pixel signals which are adjacent to the target block on the same frame. The regenerated pixel signals are the pixel signals that have been reconstructed from previously compressed pixel data. Furthermore, a differential signal which is obtained by subtracting the predictive signal from the pixel signal of the target block is encoded.
Here, in MPEG4, the pixel signal of the target block is subjected to predictive encoding after performing a discrete cosine transform on the pixel signal of the target block. In other words, for the DC component coefficient of the target block and the AC component coefficient of the first row or first column of the target block, a coefficient for the corresponding component of the block above or to the left of the target block is taken as the predictive value, and the difference between the both values is encoded. The determination of the predictive values is carried out on the basis of the size of the gradient of the DC components of a block diagonally above the target block and a block above the target block or to the left thereof. Such an intra frame predictive method appears in Patent Document 1 below.
However, with H.264, a method of generating a predictive signal by extrapolating regenerated pixel values which are adjacent to the target block in a predetermined direction is adopted. The generation of an intra frame predictive signal in this pixel domain has the advantage of making it possible to predict the details in the image. (a) of FIG. 14 shows a schematic diagram serving to illustrate the intra frame predictive method used by H.264 and (b) of FIG. 14 shows the direction for extending the pixel signal of the intra frame predictive method of H.264. In (a) of FIG. 14, block 901 is the target block and blocks 902 to 904 are adjacent blocks, where these blocks contain pixel signals which have been regenerated in previous processing. Here, a regenerated pixel group 905 which is adjacent to the target block boundary of block 901 is used to generate a predictive signal in the nine directions which are shown in (b) of FIG. 14. For example, in the case of direction “0”, a predictive signal is generated by extending the adjacent pixel directly above block 901 downward; in the case of direction “1”, a predictive signal is generated by extending the regenerated pixel to the left of block 901 to the right and, in the case of direction “2”, the average value of all of the pixel values of the pixel group 905 is generated as a predictive signal. A more specific method when generating a predictive signal appears in Non-Patent Document 1 hereinbelow, for example. With H.264, the difference between each of nine predictive signals generated in this manner and a pixel signal for the target block is taken and the predictive signal generation method with the smallest difference value is taken as the optimum predictive method (also referred to as a ‘mode’).
When transmitting image data, it is necessary to transmit identification information which indicates the optimum predictive method determined in this manner for the recovery of image data to the transmission side. Here, the predictive method determined for the two blocks, namely block 902 and block 903, is taken as the reference and information relating to the optimum predictive method of block 901 is encoded. In other words, the identification information for the method of predicting block 902 and the identification information for the method of predicting block 903 are compared and the identification information with the smallest value is determined as the reference mode information. Further, the identification information relating to the optimum method of predicting the target block is encoded relatively from this reference mode information.    Patent Document 1: U.S. Pat. No. 6,148,109    Non-Patent Document 1: Iain E. G. Richardson, “H.264 and MPEG4 video compression”, Wiley 2003, pages pp. 177 to 183.